Research over the years has explained the relation between physical fitness and insulin efficacy. Physical weakness leads to insulin resistance. When the muscles are fit and strong, they don't need as much insulin.
In fact, a person who may be lean and close to their ideal body weight may need an average only 40 to 45 units a day. However, obese person may require 80, 100, even 200 units of insulin a day.
Exercise and physical activity dramatically decrease your body's need for insulin. Muscle does better the more fit it is, the stronger it is, and the less insulin it needs. Physical activity decreases the risk to develop diabetes.